FIG. 4 shows a construction of audio signal recording and reproducing apparatus adopting a differential PCM system according to the prior art.
In FIG. 4, an input microphone 1 converts sound into an electric signal, that is, an audio signal. As input signal amplifier 2 amplifies an audio signal from the input microphone 1. An input filter 3 removes extra audio signal band frequency components among the signal from the input signal amplifier 2 and executing preemphasis thereto. An analogue to digital converter 4 digitizes the signal from the input filter 3. A new input data register 5 holds the digital data obtained by the analogue to digital converter 4. An old input data register 6 holds the A-D converted digital data of one sample prior. An input data difference operator 7 operates a difference between the data held at the both data registers 5 and 6. A memory interface 8 conducts processes such as addressing, writing in, reading out, and refresh operation against a semiconductor memory. The operation result obtained by the input data difference operator 7 is written into a semiconductor memory 9 through the memory interface 8.
An old output data register 11 holds the output data of one sample prior at the reproduction and an output data difference operator 10 reads out the difference data recorded in the semiconductor memory 9 through the memory interface 8 and conducts addition operations against the content of the old output data register 11. A new output data register 12 holds the operation result of the output data difference operator 10 and a D-A converter 13 converts the digital data at the new output data register 12 to an analogue value. An output filter 14 removes quantization noise and executing emphasis thereto. An output signal power amplifier 15 amplifies the signal from the output filter 14. A speaker 16 converts the electric audio signal from the power amplifier 15 to sound.
The device operates as follows.
At first, sound is converted into an electric signal by the input microphone 1, and thereafter the audio signal is amplified by the input signal amplifier 2 and removal of extra audio signal band frequency components and preemphasis are executed thereto by the input filter 3. Thereafter, it is digitized by the A-D converter 4. The digitized audio signal is held by the new input data register 5 and a difference between that and the data of the old input data register 6 is operated by the input data difference operator 7, and the result is written in into the semiconductor memory 9 through the memory interface 8. The data of the new input data register 5 is transferred and stored to the old input data register 6 after the conclusion of operation by the input data difference operator 7.
The difference data stored in the semiconductor memory 9 is read out by the memory interface 8 and a difference operation is executed to the old output data register 11 by the output data difference operator 10. The operation result is stored in the new output data register 12 as well as stored at the old output data register 11.
The output digital data stored at the new output data register 12 is converted into an analogue signal by the D-A converter 13 and removal of quantization noise and emphasis are executed thereto by the output filter 14. Thereafter, the result is power amplified by the output signal power amplifier 15 and is output as sound from the output speaker 16.
In the prior art differential PCM system audio signal recording and reproducing apparatus of such a construction, it is not possible to obey the change of the audio signal exceeding a predetermined range of difference value and this results in deteriorated sound quality of reproduced sound due to the compression of audio information.